In order to reduce reflections, improve signal quality, and supply the greatest power from a source to a load, the impedance "looking into the load" from the source should match the output impedance of the source. Additionally, since a mismatched line has different properties at different frequencies, a mismatched circuit is generally unsuitable for broadband or multi-frequency use. Consequently, relatively broadband circuit applications are particularly needful of impedance matching circuits.
Impedance matching is discussed in The Art of Electronics, Paul Horowittz and Winfield Hill, Cambridge University Press, Second Edition, New York, 1989 pp. 879-882 and in R. E. Collin, "Foundations For Microwave Engineering", McGraw-Hill, 1992.
Matching the impedance of a load having a high reflection coefficient is particularly difficult, sometimes requiring complex circuits that are particularly difficult to implement using conventional technologies. For example, in order to produce a matching circuit having the appropriate impedance, a conductor line on a printed circuit board may need to be thinner than may be accommodated by conventional photoligthographic techniques. The term, "a load having a high reflection coefficient" refers to a load that has a high reflection coefficient with respect to a conventional fifty-ohm system impedance. Additionally, the terms load and source may be interchanged, depending upon one's perspective so that, for example, an antenna and an RF tag circuit may be respectively viewed as source and load, or load and source, depending upon perspective. Related applications and issued patents
Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,528,222; 5,550,547; 5,552,778; 5,554,974; 5,538,803; 5,563,583; 5,565,847; 5,606,323; 5,521,601; 5,635,693; 5,673,037; 5,682,143; 5,680,106; 5,729,201; 5,729,697; 5,736,929; 5,739,754; and 5,767,789. Patent applications assigned to the assignee of the present invention include: application U.S. Pat. No. 5,673,037; Ser. No. 08/621,784, filed on Mar. 25, 1996 entitled, "Thin Radio Frequency Transponder with Leadframe Antenna Structure" by Brady et al. (pending); application Ser. No. 08/626,820, Filed: Apr. 3, 1996, entitled, "Method of Transporting RF Power to Energize Radio Frequency Transponders", by Heinrich et al.; application Ser. No. 08/694,606 filed Aug. 9, 1996 entitled, "RFID System with Write Broadcast Capability" by Heinrich et al.; application Ser. No. 08/681,741, filed Jul. 29, 1996 entitled, "RFID Transponder with Electronic Circuit Enabling and Disabling Capability", by Heinrich et al.; application Ser. No. 08/592,250 (See also PCT International Application No. PCT/EP95/03903 filed Sep. 20, 1995, and U.S. application Ser. No. 08/330,288 filed Oct. 27, 1994, now abandoned, on which the PCT application is based); U.S. Pat. No. 5,729,201; application Ser. No. 08/909,719; application Ser. No. 08/621,784; application Ser. No. 08/660,249; application Ser. No. 08/660,261; application Ser. No. 08/790,640; application Ser. No. 08/790,639; and application Ser. No. 08/681,742. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. Additionally, Patent Applications entitled, "Radio Frequency Identification Transponder Having a Spiral Antenna", "Radio Frequency Identification Transponder Having a Helical Antenna", "RFID Transponder Having Improved RF Characteristics", and "Radio Frequency Identification Transponder Employing a Patch Antenna", filed on the same day as this application and assigned to the same assignees as this application is assigned are also hereby incorporated by reference.
The applicants claim priority under 35 U.S.C. 119 (e) for provisional applications having attorney docket numbers YO897 660P1, YO897-661P1, and YO997-038P1, respectively filed on March 16, 17, and 13.